Cylinder head temperature sensor?

I'm not getting an error, I'm getting inaccurate temperature readings, mostly in the 30s, when it should be in the 70s.

I'm measuring pool water temperature. I chose thermocouples because you can easily buy a thermocouple temperature probe that you can thread into a pipe and has an enclosure at the top for the wiring. And can be outdoors. See attached picture for an example.

Thermocouple Probe.jpg